Unraveling the grain size effect on deformation and strengthening in TWIP steel through ECCI analysis

The present study investigated the effect of grain size on the deformation behavior and strengthening mechanisms of twinning-induced plasticity (TWIP) steels. Ultrafine-grained (UFG, d=0.92 μm) and coarse-grained (CG, d=11.82 μm) samples were fabricated by multi-pass cold rolling and short-term annealing. Interrupted tensile tests were employed to fabricate samples at various strain levels, and subsequent microstructural observations were conducted using characterization techniques, including electron channeling contrast imaging (ECCI). The ultrafine-grained TWIP steel with an average grain size of 0.92 μm exhibited excellent mechanical properties, which can be attributed to the formation of abundant nanoscale deformation twins and the synergistic activation of multiple deformation mechanisms during plastic deformation. Although grain refinement suppresses the formation of deformation twins, the ultra-fine grain structure leads to a sharp increase in grain boundary density, enabling deformation twins to nucleate preferentially at these boundaries. Multiple deformation mechanisms—including deformation twinning and dislocation slip-were progressively activated with increasing strain. Consequently, the dominant strengthening mechanism of the UFG sample transitioned from dislocation strengthening to twinning-induced strengthening. This transition is attributed to the promotion of nanoscale deformation twins facilitated by grain refinement. The finding may provide new insights into the strengthening and toughening mechanisms of high-manganese austenitic steels through microstructural engineering.